Page:Scientific Memoirs, Vol. 1 (1837).djvu/102

manner the plates of different crystallized bodies are combined, in order to examine the positive or negative character of their axes.

If, instead of white, either homogeneous or dichromatic light is to be made incident, small rings of wood one inch in diameter, with coloured glasses, must be fastened before the aperture of the polarizing prism $e$. When the concentrated light of a lamp giving white light falls upon dichromatic glasses, they exhibit with biaxal crystals different optical axes for the various colours, and with uniaxal crystals they yield beautiful changes of differently coloured rings. Blue glasses, which separately transmitted the extremes of the spectrum, exhibit (in arragonite, for instance,) the inner curve divided into two particoloured vacant spaces and corresponding changes within each ring; on the contrary, the two inner lings in the Iceland spar are exhibited of a deep red surrounded by violet rings gradually passing more and more into each other, during which, lighted by a flame of spirits of wine coloured by chloride of strontium, the three inner rings are violet, to which three red ones then succeed, and so forth. Through a ruby glass we now obtain only a very homogeneous red, then dark rings, in the red field of view.A flame of spirits of wine coloured yellow with common salt, or nitrate of soda, yields the most beautiful phænomenon. The dark rings and the junction-curves of the different systems of rings of twin-crystals of arragonite then appear in the linear and circular light with the utmost distinctness. For blue and violet it is best to employ the colours of the spectrum. The condensing-lens is then removed, in order that the light may fall directly upon the aperture of the polarizing prism.

The apparatus shown in Plate II. fig. 3. serves to analyse the light by reflexion, and is screwed into the pillar $s_{5}$ instead of the analysing prism. The screw at $u$ holds a concave lens of an equal focal distance. The unbordered mirror inclined at the angle of polarization is 7/4 inch long and 5/4 inch wide. A line is drawn over the three parts of the hinge $q$ on the left side of fig. 1. If the parts of this line form one straight line, the rod $bc$ is inclined towards a horizontal mirror at the angle of polarization. If $k$ and $v$ are placed aside, the light polarized by reflexion may be analysed either linearly by the prism, by the mirror in $u$, or circularly by means of $f$. But in order to examine larger cooled glasses in circularly polarized incident light, I employ a larger lamina of mica than that in $g$, which may be called $g_{\prime }$, and which fixed to the screw of the condensing-lens $p$ is screwed directly upon a wooden ring of 2 inches internal diameter. The axis of this mica lies like that of the thin plate in $g$, which is turned aside. The concave lens in $u$ is taken out, and the stand supporting the cooled glasses is brought to the distance most suitable to the eye. By holding the glasses in the hand, the various phænomena of the linear and circular light may be observed without alteration of the apparatus. If the glass be held